In the completion of wells for the production of fluids from subterranean formations, a work string is typically used. This work string may be used for drilling during which drilling muds, such as oil-based drilling muds, water-based drilling muds, and the like as well as completion fluids may have been passed through the work string. The work string may also have been used to position cement in an annulus between the outside of the casing and the inside of the well bore. The cement slurry is typically injected into the formation annulus after displacement of the drilling mud from the work string into the annulus. As a result of making up and breaking down the work string during the operations performed to complete the well, pipe thread lubricants, pipe dope and the like may also be found inside the work string. As a result any or all of drilling mud residue, cement slurry residue, oxidation products, pipe dope, pipe threat lubricants, inorganic scale materials, and other organic contaminants may be present in the work string.
After a well is complete, it is desirable to clean these materials from the work string. This is necessary since in future operations, such as the positioning of gravel packs and the like, it is highly undesirable that these materials be passed from the work string as contaminants into the gravel pack.
The use of acidic materials, such as aqueous hydrochloric acid, to pickle the work string is effective to remove some but not all of these components; specifically these acidic materials are effective in removing inorganic cement slurry scale and iron oxidation products, but are relatively ineffective with respect to pipe thread lubricants, pipe dope and any other organic materials which may be present.
Similarly, organic solvents which may be effective to remove the pipe thread lubricants, pipe dope and other organic materials are ineffective to remove other materials, such as inorganic cement scale, internal corrosion and the like.
It is highly desirable that a method and a composition be available for cleaning the interior of a work string by a single-pass circulation of a cleaning composition into and recovery of materials from the work string. Considerable effort has been directed to the development of such a method and composition.